Variable leakage transformer and control circuit therefore

ABSTRACT

A variable leakage transformer having a closed main magnetic path and a closed submagnetic path having a leg common with said main magnetic path. A primary winding is wound on said common leg, and a secondary winding is wound on said main magnetic path. The magnetic flux in the common leg is controlled by other control windings which are connected to a control DC power source. The primary winding has two portions, and one of them is closely coupled magnetically with the secondary winding in order to improve the output waveform. By controlling the magnetic flux in the sub-magnetic path, the leakage of the flux induced by the primary winding from the main magnetic path to the sub-magnetic path can be controlled; thus the coupling between the primary and the secondary winding, and conduction period in each cycle of the AC output voltage are controlled. The control of the conduction period in each cycle provides control of the power transmitted from the primary winding to the secondary winding. Because the core is made of ferrite, the transformer can be utilized at high frequency, and the size of the transformer can thus be reduced.

RELATED APPLICATIONS

This application is a continuation-in-part of prior co-pending application Ser. No. 884,953, filed Mar. 9, 1978 now U.S. Pat. No. 4,213,084, and is also a continuation-in-part of prior copending application Ser. No. 081,339, filed Oct. 3, 1979, the same being in turn a continuation-in-part of the aforesaid application Ser. No. 884,953, all of said applications being assigned to the same assignee.

BACKGROUND OF THE INVENTION

The present invention concerns the structure and application of a variable leakage transformer or a variable voltage transformer.

A transformer has, in general, a magnetic core having a closed magnetic path, a primary and secondary windings wound on said magnetic core, and all the input power applied to the primary winding is provided at the output of the secondary winding except for a small amount of loss in the transformer. In this case, the output voltage V₂ across the secondary winding is

    V.sub.2 =(n.sub.2 /n.sub.1)×V.sub.1,

where V₁ is the voltage across the primary winding and n₁ and n₂ are the number of windings of the primary and secondary windings, respectively.

When we want to control the output power of the transformer, a controlable switching device such as a SCR (Silicon Controlled Rectifier) or a transistor must be employed at the output of the transformer. In said controlable switching device, the pulse width during each cycle is varied by controlling the conducting time of the switching element (SCR). However, a prior controllable switching device has the disadvantage that the circuit is very complicated and the price of the device is rather high.

Another prior art device for controlling an AC power source is a magnetic amplifier, in which a saturable reactor is inserted between the power source and the load, and by controlling the reactor, the power transferred from the source to the load is controlled. However, a magnetic amplifier has the disadvantage that the voltage across the load must be the same as that of the power source, and the saturable reactor does not function as a variable-voltage transformer.

Another prior art device for controlling an AC power source is shown in U.S. Pat. No. 1,376,978, in which a primary winding, a secondary winding, and an exciting coil are provided on the core, and by controlling the saturation level of the core by said exciting coil, the magnetic leakage between the primary winding and the secondary winding is controlled, and then, the voltage across the secondary winding is controlled. However, said U.S. patent has the disadvantage that the transformer can not be utilized for high frequency (for instance the frequency higher than 1,000 Hz) since the core of that transformer is a laminated silicon steel plate. Accordingly, when we try to make a D.C. voltage converter, we must use the low frequency oscillator and transformer, and so the size of the transformer must be large. If we try to use the transformer in that U.S. Patent in high frequency, the loss of energy in the transformer would be increased.

SUMMARY OF THE INVENTION

It is an object of the present invention, therefore, to overcome the disadvantages and limitations of prior transformers by providing a new and improved controllable transformer which operates on the principle of leakage flux control.

Another object of the present invention is to provide a variable leakage transformer which can operate in high frequency with a small loss.

It is also an object of the present invention to provide a new and improved stabilized power source using the present transformer.

The above and other objects are attained by a variable leakage transformer comprising a ferrite core with a main magnetic path and a sub-magnetic path, the main magnetic path having at least a common magnetic path with the sub-magnetic path, a primary winding wound on said common magnetic path, a secondary winding wound on said main magnetic path, and means for controlling the magnetic flux in said sub-magnetic path.

Preferably, the magnetic flux in said sub-magnetic path is controlled by the direct current in the control winding wound on said sub-magnetic path.

Preferably, said primary winding has a first primary winding portion wound on the common magnetic path, and the second primary winding portion wound on the main magnetic path and being closely coupled magnetically with the secondary winding.

In the particular application of the present transformer for a stabilized power source, the direct current in said control winding reflects the change of the output voltage across the secondary winding.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and attendant advantages of the present invention will be appreciated as the same become better understood by means of the following description and accompanying drawings wherein;

FIG. 1 is the cross sectional view of the present transformer,

FIG. 2(A), FIG. 2(B) and FIG. 2(C) show the operational waveforms of the transformer in FIG. 1,

FIG. 3(A) and FIG. 3(B) show the other operational waveforms of the transformer in FIG. 1,

FIG. 4 is the cross sectional view of another embodiment of the present transformer,

FIG. 5 is the cross sectional view of another embodiment of the present transformer,

FIG. 6 is the perspective view of still another embodiment of the present transformer,

FIG. 7 is the plane view of the transformer in FIG. 6,

FIG. 8 is the side view of the transformer in FIG. 6,

FIG. 9 shows the structure of the core for the use of the transformer in FIG. 6,

FIG. 10 is the circuitry of the stabilized power source using the present transformer,

FIG. 11 is the embodiment of the circuitry of the control circuit of the power source in FIG. 10,

FIGS. 12A-12D show the operational waveforms of the circuit shown in FIG. 10,

FIGS. 13A-13D show the other operational waveforms of the circuit shown in FIG. 10,

FIG. 14 is the circuit of another stabilized power source using the present transformer,

FIG. 15 is the circuit of the control circuit of the power source in FIG. 14,

FIG. 16 is the circuit of still another stabilized power source using the present transformer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the first embodiment, depicting a configuration based on the principle of this invention. In this figure, the magnetic core 1, composed of the combination of either the E shaped core and I shaped cores or a pair of E shaped magnetic cores, is a three legged magnetic core having 2 magnetic paths, a sub-magnetic path AA'B'B and a main magnetic path AA'C'C. The middle leg AA' and the side leg CC' of the magnetic core 1 are provided with the primary coil 2 and secondary coil 3, respectively. The number of turns of the primary coil 2 is assumed to be N₁ and that of the secondary coil N₂.

Now, the load resistor R_(L) is connected to the secondary coil 3 and AC (Alternate Current) input voltage V_(in) is applied to the primary coil 2. The magnetic flux φ generated thereby is divided into the magnetic flux φ₁ that surrounds the sub-magnetic path AA'B'B, and the magnetic flux φ₂ that surrounds the main magnetic path AA'C'C. That is, the formula φ=φ₁ +φ₂ is satisfied. Of these two fluxes, the one that supplies power to the load resistor R_(L) is the one that interlinks with the secondary coil 3, i.e. φ₂. The φ₁ does not contribute power supply to the load resistor R_(L). At this stage, if the magnetic reluctance is increased by application of the control magnetic field H_(c) to the sub-magnetic path AA'B'B from outside, the magnetic flux φ₁ decreases with resultant proportionate increases in the magnetic flux φ₂. Therefore, the output voltage V_(out) of the secondary coil 3, i.e. the power supplied to the load resistor R_(L), increases. Eventually, it can be seen that power transferred from the primary to the secondary can be controlled by changing the strength of magnetic field through application of the control magnetic field H_(c) to the sub-magnetic path AA'B'B from outside.

Suppose that a rectangular wave voltage is applied as input voltage V_(in) such as given in FIG. 2(A), if the sub-magnetic path AA'B'B is in a perfectly saturated condition in the control magnetic field H_(c), because this sub-magnetic path is considered to be equivalent to non-existent as a magnetic circuit, the output wave form of the output voltage V_(out) thus obtained is identical with that of the input voltage V_(in) shown in FIG. 2(B). The amplitude is determined by the ratio between the number of turns N₁ on the primary coil 2 and the number of turns N₂ on the secondary coil 3. That is, V_(out) =V_(in) (N₂ /N₁).

Suppose that the sub-magnetic path AA'B'B is put in a non-saturated condition by weakening the control magnetic field H_(c), some of the magnetic flux φ generated by input voltage V_(in) flows to the sub-magnetic path during the time t₁ i.e. from the time the input voltage V_(in) is applied until the time the sub-magnetic path AA'B'B is saturated by the input voltage V_(in). Then, the output voltage V_(out) in effect becomes almost zero.

After a period of time t₁, the sub-magnetic path AA'B'B becomes saturated. This saturated condition is maintained until inversion of the input voltage V_(in). Therefore, as given in FIG. 2(C) the output voltage V_(out) (=V_(in) N₂ /N₁) appears only during the time (T/2-t₁). At this stage, the length of time t₁ can be changed by changing the magnetized condition of the sub-magnetic path AA'B'B, or in other words by the magnitude of the control magnetic field H_(c) applied to the sub-magnetic path AA'B'B.

Thus, by increasing or decreasing the magnitude of the control magnetic field H_(c), the pulse width of output voltage V_(out) can be controlled. During the time t₁, power consumption in the primary winding is zero, because the magnetic flux φ surrounds the sub-magnetic path AA'B'B and does not interlink with the secondary coil 3, and therefore, the load as viewed from the primary coil 2 is in a condition equivalent to open circuit. In other words, the control does not affect power efficiency and the operation is always highly efficient.

Performance of this first embodiment shows that the pulse width of the output voltage V_(out) can be efficiently controlled by increasing or decreasing the magnitude of the control magnetic field H_(c).

It should be appreciated of course that an input voltage of sinusoidal waveform is also applicable and provides the same effect as a rectangular waveform, although the embodiment shows the rectangular waveform for the sake of simplicity of the explanation.

As explained above, according to the present invention, the leakage of the magnetic flux from the main magnetic path to the sub-magnetic path can be controlled by varying the magnetic flux in the sub-magnetic path, and thus, the coupling between the primary and secondary windings and the conduction period in each cycle of the input voltage are controlled. And it should be noted that the control of the conduction period provides the control of the power transmission from the primary winding to the secondary winding.

In the above embodiment, the primary coil 2 was wound around the middle leg of the magnetic core 1 and the secondary coil 3 was wound around one of the side legs. As a result of this arrangement, the magnetic coupling between the primary coil 2 and the secondary coil 3 has a tendency to be insufficient. If the coupling is insufficient, even if the rectangular wave input voltage V_(in) as given in FIG. 3(A) is applied, the wave form of the output voltage V_(out) becomes deformed as shown in FIG. 3(B) and the efficiency of the transformer is decreased. This results from insufficient coupling between the primary coil 2 and the secondary coil 3.

FIG. 4 depicts the second embodiment which provides a configuration with improvements on the above mentioned drawback. In this Figure, the primary coil is divided into the first primary coil 2A and the second primary coil 2B. The first primary coil 2A surrounds the middle leg AA' of the magnetic core 1 and the second primary coil 2B surrounds the side leg CC'. The secondary coil 3 is also wound over the second primary coil 2B. With this arrangement, the coupling between the primary and the secondary coils becomes sufficient and the wave form of the output voltage V_(out) and that of the input voltage V_(in) can be made almost identical.

The configuration in FIG. 4 with the divided primary windings is advantageous in particular when the magnetic core constituting the main magnetic path and the sub-magnetic path is ferrite, whose magnetic permiability is rather small. When the magnetic permiability of the core is small (less than 10,000), the coupling between the primary winding and the secondary winding is insufficient, and the output waveform is deformed. This insufficiency is overcome by dividing the primary winding to the first primary winding and the second primary winding, one of which is closely coupled with the secondary winding, as shown in FIG. 4. Since a ferrite core is excellent in a high frequency operation, the transformer utilizing a ferrite core operating in high frequency can be constituted in a small size.

According to one embodiment of the present invention, a ferrite core with the magnetic permiability being approximate 300˜1000 is utilized, the frequency of the input signal is in the range 20˜30 kH_(z), and 50% percent of turns of the primary winding is divided to the second primary winding.

The preferable ferrite core is obtained in the commercial market as the tradename H7C1 manufactured by TDK Electronics, Co., Ltd., Tokyo, Japan, the magnetic permiability of said H7C1 is approximate 5,000 in a ring shaped core, and is approximate 300˜1000 in an assembled E and I cores. The permiability in an assembled E and I cores is considerably decreased compared with that of a ring shaped core, because of the inevitable air gap at the coupling portion between an E shaped core and an I shaped core.

When we make a DC-DC converter utilizing the present leakage pulse transformer, the size of the transformer can be considerably reduced by operating the transformer in high frequency. And a ferrite core is effective in making possible the operation of the transformer in high frequency.

FIG. 5 illustrates the 3rd embodiment. In this figure, the middle leg AA' of the magnetic core 1 is provided with the primary coil 2, and the side leg CC' with the secondary coil 3. At the same time, the other side leg BB' is provided with the control magnetic core 5 surrounded by the control coil 4. The control magnetic core 5 is an E shaped magnetic core with its middle leg D designed to be surrounded by the control coil 4.

In the above arrangement, passage of the control current I_(c) through the control coil 4, causes the generation of a control magnetic field on the control coil 4. This in turn generates: (1) a control magnetic flux φ that passes through the middle leg D of the control magnetic core 5, the side leg BB' of the magnetic core 1 and to the side leg F of the control magnetic core 5; and (2) a control magnetic flux φ_(c) that passes through the middle leg D, side leg BB' and to the side leg G. At this stage, if the control current I_(c) is increased or decreased, the control magnetic flux φ_(c) also increases or decreases proportionally. Therefore, if the control current I_(c) is increased, the magnetic reluctance of the sub-magnetic path AA'B'B increases and the resultant condition is susceptible to magnetic saturation. Therefore, if rectangular waves such as shown in FIG. 2(A) are applied as input voltage V_(in), part of the sub-magnetic path AA'B'B is saturated in a short time with less magnetic flux φ₁. Consequently, the time t₁ indicated in FIG. 2(C) becomes short, while the pulse width of output voltage V_(out) becomes long. Conversely, if control current I_(c) is decreased, magnetic reluctance of the sub-magnetic path AA'B'B becomes small and possibility of magnetic saturation becomes remote. Then, it will take time to saturate part of the sub-magnetic path. Consequently, the time t₁ shown in FIG. 2(C) becomes long and the pulse width of output voltage V_(out) is shortened.

The third embodiment in FIG. 5 is capable of controlling pulse width of the output voltage V_(out) very efficiently through increase or decrease of control current I_(c). Consequently, the power transferred to the load resistor R_(L) from the input can be electrically controlled. The fact is that the direction of the magnetic flux φ₁ surrounding the sub-magnetic path AA'B'B is inverted following the input voltage V_(in). However, since control magnetic flux φ_(c) is constantly flowing in both directions, either one of the magnetic fluxes φ_(c) and the magnetic flux φ₁ always join together and cause the magnetic saturation. Therefore, control current I_(c) can be direct current and is readily controllable.

FIGS. 6, 7 and 8 illustrate the fourth embodiment. In these figures, the magnetic core 1A forming the main magnetic path is made up of the combination of the U shaped core and the I shaped core, and the magnetic core 1B forming the sub-magnetic path is made up of the combination of the E shaped core and the I shaped core. And, the primary coil 2 is wound in common around one of the legs of the magnetic core 1A and the middle leg of the magnetic core 1B while the secondary coil 3 is wound around the outer leg of the magnetic core 1A. Both the side legs of the magnetic core 1B are provided with the control coils 4A and 4B. The control coils 4A and 4B are connected in series so that the voltage induced in these coils will cancel each other when the input voltage V_(in) is applied to the primary coil 2.

According to the above arrangement, the magnetized condition of the magnetic core 1B forming the sub-magnetic path can be changed by the control current I_(c) fed to the control coils 4A and 4B. That is, when the rectangular wave such as shown in FIG. 2(A) is applied as input voltage V_(in), the magnetization strength of the sub-magnetic path becomes large if control current I_(c) is large, while the time t₁ in FIG. 2(C) becomes short. If the control current I_(c) is small, the strength of magnetization of the sub-magnetic path becomes weak while the time t₁ becomes long. As a result, pulse width of output voltage V_(out) can be controlled.

In the fourth embodiment in FIG. 6 the magnetic core is made up of the EI magnetic core and the UI magnetic core combined. However, it should be appreciated that a magnetic core having the same effect may be formed by a combination of the four-legged magnetic core IC and the T shape magnetic core ID as depicted in FIG. 9.

Although FIG. 6 disclosed the embodiment using U-I core and E-I core, it should be appreciated of course that many modifications of FIG. 6, for instance the embodiment using U--U core and E--E core, are possible.

In the third and fourth embodiments shown in FIGS. 5 and 6, the wave form of output voltage V_(out) can be improved by dividing the primary coil 2 as demonstrated in the second embodiment and by closely coupling a part of the primary coil to the secondary coil 3.

FIG. 10 depicts the embodiment of the stabilized D.C. power source using the present transformer. In this drawing, the primary winding N₁ of the transformer 21 is a so-called bifilar coil with a center tap. To this primary winding N₁ is connected the self-exciting type push-pull converter 23 incorporating the transistors Q₁ and Q₂ connected to the push-pull converter as well as the oscillation transformer 22. And, the direct current power source 24 is connected between the center tap of the primary winding N₁ and the emitter of the transistors Q₁ and Q₂ so that the power is fed to the converter 23.

The secondary winding N₂ of the transformer 21 is also provided with a center tap. The full-wave rectification circuit 25 composed of the rectification diode D₁, D₂, reactor L and capacitor C₁, is connected to the secondary winding N₂. The DC output voltage V_(out) of this full-wave rectification circuit 25 is fed to the load 26. The aforementioned pulse width control transformer 21 is equipped with the auxiliary supply winding N₃ and control winding N₄. The operation voltage V_(c), AC output of the auxiliary supply winding N₂, rectified through rectification diode D₃ and capacitor C₂, is applied to the control circuit 27. The control circuit 27 detects changes in the aforementioned output voltage V_(out) and controls the value of the control current I_(c) to be fed to the control winding N₄.

It should be appreciated in FIG. 10 that the transformer 21 is a variable leakage transformer according to the present invention, and the push-pull converter 23 provides the input voltage of the rectangular waveform to the primary winding N₁ of the transformer 21.

As illustrated in FIG. 11, the control circuit 27 is equipped with a series circuit consisting of, the transistor Q₃ which feeds control current I_(c) to the control winding N₄, upon receiving the operation voltage V_(c), the V_(c) being rectified current of AC output of the aforementioned auxiliary supply winding N₃, and the resistor R₁. The control circuit also has the zener diode ZD that detects output voltage V_(out) of the aforementioned full-wave rectification circuit 5, and the transistor Q₄ to be put into operation by ZD.

The above mentioned control circuit 27 lets the current pass through the transistor Q₄ by the break-down of the zener diode ZD when the output voltage V_(out) exceeds the zener voltage of the zener diode ZD, and, reduces emitter current i.e. control current I_(c) of the transistor Q₃.

In the configuration of the FIGS. 10 and 11, if the sub-magnetic path of the transformer 21, is sufficiently saturated by having the prescribed control current I_(c) passing through the control winding N₄, the sub-magnetic path becomes equivalent to non-existent. The P-P' voltage of the selfexciting push-pull type converter 23 assumes a rectangular wave form as indicated in FIG. 12(A). The collector current I₁ of the transistor Q₁ and the collector curent I₂ of the transistor Q₂ assume wave forms as depicted in FIG. 12(B) and 12(C) respectively. Therefore, the voltage induced in the secondary winding N₂, as in the case of an ordinary transformer, becomes a rectangular wave form similar to that induced in the primary winding as shown in FIG. 12(D).

In this situation, transfer of power from the primary to the secondary becomes maximum and the output DC voltage V_(out) also reaches its maximum value.

When the terminal voltage of the DC power source 24 rises, or the load 26 decreases resulting in an increase in the output DC voltage V_(out), the current flows through the zener diode ZD in the control circuit 27 and the base bias current of the transistor Q₃ is divided by the transistor Q₄. As a result, emitter current of the transistor Q₃, i.e. control current I_(c) decreases and the magnetic reluctance of the magnetic core, of the sub-magnetic path of the transformer 21, drops. Then, most of the magnetic flux generated by the primary winding N₁ flows to the sub-magnetic path and reduces the coupling with the secondary winding N₂. Therefore, even if the P-P' voltage of the self-exciting push-pull type converter 23 assumes the rectangular wave form indicated in FIG. 13(A), the collector current I₁ of the transistor Q₁ and the collector current I₂ of the transistor Q₂ assume wave forms as shown in FIG. 13(B) and 13(C) respectively. That is, until the time t₁ when the sub-magnetic path has been magnetically saturated by the magnetic flux established by the primary winding N₁, only very little current corresponding to the exciting current for exciting the magnetic core of the transformer 21, flows as collector current of each transistor. After a period of time t₁, the sub-magnetic path is saturated and the magnetic flux of the primary winding N₁ couples with the secondary winding N₂. Therefore, the voltage appearing in the secondary winding N₂ becomes a bipolar type pulse such as shown in FIG. 13(D). That is, compared with the FIG. 13(D) the pulse width becomes shorter by as much as the time duration t₁ and therefore, in response thereto the output DC voltage V_(out) drops.

Thus, in the embodiment of FIG. 13, the output DC voltage V_(out) can be constantly maintained at a given level, if the control current I_(c) and the characteristics of the zener diode ZD are appropriately set contingent upon the output DC voltage V_(out) desired. In this situation, the collector currents of the transistors Q₁ and Q₂ of the aforementioned converter become only exciting current at the time t₁, and the output end of the converter 23 reaches an open circuit or no-load condition. Therefore, the control does not cause a power loss, while conversion efficiency of the converter 23 scarcely decreases. In the conventional pulse control system, in order to control operation of a switching element, a feed back circuit from the secondary to the primary of the transformer is necessary. However, according to the above embodiment, pulse width control can be accomplished on the secondary side of the transformer 21, and the primary is irrelevant to the control. Therefore, a high dielectric strength as prescribed by various safety standards can be readily provided between the primary and the secondary. Further, since a complex circuit arrangement is not required, a cost reduction opportunity is available.

The embodiment described above depicted application in a situation where stable output DC voltage is supplied to the load. The another embodiment is given in FIG. 14, where the load generated by lamps and heaters is directly driven by AC current. In this figure, the output of the converter 23 is applied to the primary winding N₁ of the transformer 21A, and the load 26A is connected to the secondary winding N₂ of the transformer 21A. The AC output of the auxiliary supply winding N₃ of the transformer 21A is rectified by the rectification diode D₃ and the capacitor C₂. This operational voltage V_(c) is then supplied to the control circuit 27A. The control circuit 27A detects the changes in the bipolar-type pulse output AC voltage V'_(out) applied to the load 26A, and controls the value of the control current I_(c) to be fed to the control winding N₄.

In addition to the configuration of the control circuit 27 depicted in FIG. 10, the control circuit 27A, as shown in FIG. 15, is provided with the 1'st series circuit composed of the bridge rectifier 28, capacitor C₃ and the resistor R₃ as well as the second series circuit composed of the capacitor C₄ and the resistor R₄. With this arrangement, the DC value, directly proportional to the effective value of the output AC voltage V_(out), is output at both ends of the 2'nd series circuit and is applied to the zener diode ZD. That is, the control circuit 27A applies to the control winding N₄ the control current I_(c) that corresponds to the effective value of the output AC voltage V'_(out).

With respect to the embodiment mentioned above, the effective value of the output AC voltage V'_(out) can be constantly maintained at a given level if the characteristics of each element, the control current I_(c) etc., in the control circuit 27A are appropriately set with close correlation to the desired output AC voltage V'_(out). Thus, the performance is similar to the embodiment in FIG. 10.

So far, description of the operation has been concerned with situations where a single stabilized output is to be obtained. There are situations where multiple outputs are required and each of the outputs must be independently stabilized.

FIG. 16 is another embodiment of this invention with a configuration appropriate to the above situation. In this figure, output of the converter 23 is applied to the primary winding L₁ of the output transformer 30 provided with the required numbers of output windings. And, to each of the output windings L₂₁, L₂₂ and L₂₃ is connected each of the primary windings N₁₁, N₁₂ and N₁₃ respectively of the control transformers 21X, 21Y and 21Z respectively. Each of the control windings N₄₁, N₄₂ and N₄₃ of the control transformers 21X, 21Y and 21Z respectively is provided with the control circuits 27X, 27Y and 27Z respectively. With these control circuits, the control of the output pulse width of the secondary windings N₂₁, N₂₂, N₂₃ of the control transformers 21X, 21Y and 21Z respectively is effected: i.e. regulation of output voltage is accomplished.

According to the configuration described in FIG. 16 above, a single converter 23 can be used for each stabilized output in common. Compared with a situation where a converter is provided for each regulated output, this arrangement greatly simplifies the circuit configuration and, therefore, is less costly. Further, the transformer 30 can be removed. In that case, the parallel connection of primary windings N₁₁, N₁₂, N₁₃ is connected directly to the output of the converter 23.

As described above, with these embodiments, output pulse width is controlled by the control transformer. Therefore, the oscillation frequency of the converter can be arbitrarily set to meet the current demand. This facilitates the design of the converter. At the same time, control of the operation of the switching element in the converter is unnecessary. Since the feedback circuit from the secondary to the primary of the transformer is unnecessary, insulation between the primary and the secondary is facilitated. Special contrivances for a feedback circuit are not necessary either. The circuit arrangement is simple while affording economical improvement.

The converter can be a self-exciting push-pull type, a blocking oscillator type with a single transistor, separately excited converter, etc., or a converter incorporating a transistor or thyristor as the switching element. If multiple regulated output are desired, the control transformers may be connected in parallel to a single converter.

As detailed above, this invention provides a high performance regulated power supply device of a simple configuration incorporating the control transformer.

In particular, it should be appreciated that two of the important features of the present invention is (1) use of a ferrite core which can operate in high frequency, and (2) the structure of the divided primary winding for providing the sufficient coupling between the primary winding and the secondary winding when a core with a small permiability like ferrite core is utilized.

From the foregoing it will now be apparent that a new and improved transformer and a stabilized power source using the transformer have been found. It should be understood of course that the embodiments disclosed are merely illustrative and are not intended to limit scope of the invention. Reference should be made to the appended claims, therefore, rather than the specification as indicating the scope of the invention. 

What is claimed is:
 1. A variable leakage transformer, comprising:a ferrite core with (i) a closed main magnetic path and (ii) a closed sub-magnetic path having a pair of closed magnetic loops with a common leg, the main magnetic path having a portion including said common leg; a primary winding adapted to be coupled to an input power source, said primary winding having a first primary winding portion and a second primary winding portion connected in series with each other,said first primary winding portion being wound on said common leg, said second primary winding portion being wound on another part of the main magnetic path; a secondary winding wound on said other part of said main magnetic path in closely coupled magnetic relationship with said second primary winding portion and adapted to be connected to a load; and a pair of control windings each wound on one of said closed magnetic loops of said sub-magnetic path remote from said common leg, said control windings being connected in series with each other, the control windings being adapted to be connected to an input DC power source to control the magnetic flux in said sub-magnetic path and in said common leg portion of said main magnetic path.
 2. A variable leakage transformer according to claim 1, wherein the permeability of said ferrite core is less than 5,000.
 3. A variable leakage transformer according to claim 1, wherein the permeability of said ferrite core is less than 1,000.
 4. A variable leakage transformer according to claim 1, wherein the permeability of said ferrite core is less than
 300. 5. A variable leakage transformer according to claim 1, wherein the operating frequency of the transformer is higher than 20 kHz.
 6. A variable leakage transformer according to claim 1, wherein the number of turns of the first primary winding portion is the same as that of the second primary winding portion.
 7. A variable leakage transformer according to claim 1, further comprising a control core having a control winding for controlling the magnetic flux in said sub-magnetic path, said control core being inserted in said sub-magnetic path.
 8. A variable leakage transformer according to claim 1, further comprising a power supply connected to the input of said primary winding for supplying an alternating current to said primary winding, and a control circuit for detecting variation of the output voltage across said secondary winding and controlling the D.C. current in the control winding to reduce said variation.
 9. A variable leakage transformer according to claim 8, further comprising an auxiliary power supply winding wound on said core, wherein said control circuit comprises a rectifier for rectifying the output of the auxiliary winding, and means for controllably coupling the output of said rectifier to the control winding of the transformer.
 10. A transformer assembly comprising a plurality of variable leakage transformer elements each having a primary winding and a secondary winding, said primary windings being connected in parallel with one another, each of the variable leakage transformer elements being independently controllable, the primary windings of said variable leakage transformer elements being commonly supplied with an AC input voltage, means for connecting the secondary winding of each of said variable leakage transformer elements to an associated load to supply a stabilized voltage thereto, each of said variable leakage transformer elements having (i) a ferrite core with a closed main magnetic path and a closed sub-magnetic path, a portion of the main magnetic path having a common path with the sub-magnetic path, (ii) a primary winding wound on the common path of the core, (iii) a secondary winding wound on the main magnetic path of the core, and (iv) means for controlling the magnetic flux in said sub-magnetic path.
 11. A variable leakage transformer comprising a first ferrite core having the combination of an E-shaped core and an I shaped core which cooperate to form closed magnetic paths, a second ferrite core having the combination of a U-shaped core and an I-shaped core which cooperate to form a closed magnetic path, the center leg of said E-shaped core being coupled with one leg of said U-shaped core, and forming a common magnetic path, said U-shaped core and the corresponding I-shaped core forming a main magnetic path, said E-shaped core and the corresponding I-shaped core forming a sub-magnetic path, a primary winding having a first primary winding portion and a second primary winding portion connected in series with each other, a secondary winding wound on the main magnetic path of the core, a pair of control windings each wound on the said legs of said E-shaped core and being connected in series with each other, said first primary winding portion being wound on the common magnetic path, said second primary winding portion being wound on the main magnetic path and being closely coupled magnetically with the secondary winding. 